1. Technical Field
The present invention relates to liquid crystal light valves (LCLV's), and more particularly to a liquid crystal light valve employing an improved cadmium telluride light blocking layer.
2. Discussion
Liquid crystal light valves have diverse applications as light amplifiers, projectors and image processors. One prior LCLV is disclosed in U.S. Pat. No. 4,019,807 assigned to Hughes Aircraft Company, and which is herein incorporated by reference. The light valve of that patent employs successive device layers comprising a cadmium sulfide (CdS) photoconductor, a cadmium telluride (CdTe) light absorbing layer, a dielectric mirror, an insulting silicon dioxide layer and a liquid crystal layer. This device structure is sandwiched between indium-tin-oxide transparent electrodes deposited on an optical quality glass flat substrate. The particular function of the CdTe light absorbing layer is to provide several orders of magnitude of light blocking to prevent high intensity light from saturating the photoconductive layer.
U.S. Pat. No. 4,799,773 assigned to Hughes Aircraft Company discloses an improved LCLV device featuring an amorphous silicon conductive layer and a dielectric mirror layer employing titanium dioxide (TiO.sub.2). A special bonding structure is used to bond a cadmium telluride light blocking layer to the amorphous silicon. A titanium dioxide-silicon dioxide layered dielectric mirror is then applied to the cadmium telluride light blocking layer.
The titanium dioxide layer disclosed in U.S. Pat. No. 4,799,773 provides greatly improved resolution and spectral capabilities. The amorphous silicon contributes an improved photoconductor response time, approaching the speed required for raster scan displays. The special bonding structure ties the structure together and contributes to an overall improvement in repeatability. The bonding structure includes successive layers comprising first and second SiO.sub.2 layers and first and second CdTe blocking layers. The adjacent SiO.sub.2 and CdTe layers are oxygen enriched and the device, including the bonding structure, is annealed prior to application of a CdTe layer to redistribute the oxygen. U.S. Pat. No. 3,824,002 assigned to Hughes Aircraft Company, also discloses the use of CdTe as a light blocking layer. Both U.S. Pat. Nos. 4,799,773 and 3,824,002 are herein incorporated by reference.
Most commonly, the cadmium telluride light blocking layer is applied using an evaporation process. However, for high throughput applications, evaporation is not suitable because it is too slow, the equipment is costly, labor and overhead are high, and the vacuum must be broken during the manufacturing process. Thus, it would be desirable to provide an alternative to the evaporation process for producing a cadmium telluride light blocking layer in LCLVs in high volume applications.
Sputtering is an alternative process which avoids many of the disadvantages of evaporation. Sputtering improves throughput, requires less costly equipment, requires lower labor and overhead, and does not require breaking vacuum during the process. However, the performance of LCLV's using sputtered cadmium telluride light blocking layers have not been entirely satisfactory. This is because of a phenomenon called "photoshading". Photoshading results in uneven output brightness across the area of the LCLV. It appears to be related to an instability in photoconductivity, since it is not immediately apparent but only manifests after a burn-in period.
The causes of photoshading are not completely understood. However, the phenomenon does seem to be related to the CdTe sputtering target. That is, photoshading is much worse with some targets than others. However, with high-volume, cost-sensitive applications, it is impractical to use only certain targets, particularly since it is not certain in advance which targets will produce photoshading and which will not.
Accordingly, it would be desirable to provide a technique for sputtering cadmium telluride light blocking layers in LCLVs which yields stable devices that do not exhibit photoshading. It would further to be desirable to provide such a technique which is inexpensive and easily implemented for high-volume applications. Also, it would be desirable to provide a technique for sputtering cadmium telluride light blocking layers which can tolerate normal variations in cadmium telluride targets and which does not produce photoshading with certain targets. Further, it would be desirable to provide such a technique which results in low particulate on the light blocking layer.